Gneiss (Granitic Gneiss)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Banded pattern of pink (feldspar), white/gray (quartz), and black (biotite/amphibole); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks. It is subjected to intense heat and pressure deep within the Earth's crust, causing minerals to recrystallize and segregate into bands.

Uses & Applications

Used widely as a crushed stone in construction (road base, concrete aggregate), dimensional stone for flooring and facades, and as decorative landscaping or gravestones.

Geological Facts

Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The term 'gneiss' has been used in English since at least 1757.

Field Identification & Locations

Identify in the field by looking for 'gneissic banding'—alternating layers of light and dark minerals that do not easily split (unlike schist). Commonly found in the cores of mountain ranges and ancient continental shields.

Identified on: 4/25/2026

Mode: Standard